1. Field of the Invention
This invention relates to method and apparatus for preventing unauthorized copying of original documents by conventional electrostatic photocopiers and, more particularly, to a metalized printable substrate having a screened geometric pattern printed on the substrate to form a diffraction grating for producing a random interference pattern of diffracted light when the substrate is illuminated by a photocopier light source to render a copy of the printed information illegible.
2. Description of the Prior Art
Recent advancements in the development of sophisticated photocopying machines, particularly the color copier, has compromised document security by permitting unauthorized duplication of original documents for the purpose of passing them off as original documents. This problem is particularly acute in the manufacture of forged and counterfeit valuable documents, such as currency and other negotiable instruments. The proliferation of this problem has generated many devices and methods to prevent unauthorized copying, forgery and counterfeiting by security measures that reveal a violation at a glance.
Prior to the development of the color copier, optically invariable devices (OIDs) were effective to deter unauthorized copying and counterfeiting. OIDs use yellow or blue printing which does not reproduce on older black and white photocopiers. However, optically invariable devices have been compromised by the advent of advanced analog and digital color copiers. To counter these measures optically variable devices (OVDs) have been developed and are based on specular reflection (metallic foils), diffraction (holograms, etc.) or interference (thin films, etc.).
Optically invariable devices are generally diffusely reflecting devices which are independent of the angle of illumination and observation. It is generally known that OIDs are copied easily by advanced color copiers and other four color reproduction systems. However, optically variable devices were developed to deter four color copy fraud. OVDs, such as embossed holograms and iridescent devices, include images or patterns which appear to change as the angle of light or vision changes. Overall OVDs are based on either specular reflection (metallic foils), diffraction (holograms, kinegrams, pixelgrams, etc.) or interference (thin films, Bragg structures and liquid crystal).
Security documents which rely on diffuse color printing are printed with the color images by diffuse pigments which are only ordered in the visual millimeter scale. For example, information is printed or typed on paper with color having a reflection spectral response of less than 10% for light with a wavelength below about 600 millimicrons. The color is sufficiently contrasting with the information to enable the information to be read by the human eye when the document is viewed under white light, but the document cannot be successfully copied by an electrostatic photocopier. U.S. Pat. No. 4,522,429 is representative of this type of antiphotocopying paper.
Similarly, U.S. Pat. No. 4,632,429 discloses a document having a portion being capable of transmitting visual light from the rear face to the front face to cause sufficient contrast between the relatively non-translucent information and the transmitted light. The information can be read by the human eye viewing the front face of the document when visible light is transmitted through the document from the rear face to the front face thereof. The front face portion has a color with a sufficiently low reflection spectral response to render the document portion substantially incapable of being photocopied in an information-readable manner.
U.S. Pat. No. 5,169,722 discloses an anticopying composite film on a paper substrate which acts as a mirror when placed upon the platen of a photocopier of the 0/45.degree. type, which utilizes specular illumination rather than diffuse illumination. The resulting copies turn out black and mask the type or print thereon. The anticopying composite film includes a substratum of paper of synthetic resin film, a metallic foil formed on the surface of the substratum, and a colored transparent synthetic resin coating applied to the surface of the metallic foil. With this arrangement, the printing cannot be reproduced when the information is photocopied.
In another arrangement, as disclosed in U.S. Pat. No. 5,290,068, a lacquer is applied to a paper surface coated with a layer of a colorless, color developer. A gloss is added to the surface of the paper and provides specular reflection characteristics that enhance the uncopiability of the final product.
It is also well known to apply a watermark or a continuous tone image to a paper surface. Under light the diffused reflection of the watermark is visible as a negative image because the thick portions of the paper have a higher diffuse reflection than the thin portions. Overall, however, it is recognized that the essential continuous tone properties of the watermark appear to be difficult to communicate, requiring holding the security document against the light in order to view the watermark.
Microtext is a similar approach to creating a security document. Microtext consists of very small lettering, beyond the 300-600 d.p.i. resolution of the usual scanner and color and black and white copiers. One disadvantage of the microtext is that a message consisting of 0.03 mm letters is below the resolving power of human vision and its verification requires a magnifier. It is also known to use fine line security patterns to deter counterfeiting. The fineness of the lines is beyond the resolution of the usual scanner.
Intaglio printing is a well known method for imprinting security images into documents, as disclosed in U.S. Pat. No. 5,487,567. Intaglio printing consists of tactile raised lines of which the ink is transferred to the document under high printing pressures. The latent image consists of a fine intaglio pattern defined by image portions which differ in depth orientation from other elements forming the background. The ability to discern the image portion from the background varies noticeably depending upon the angle of view and the orientation of the document. Under oblique observation, foreground and background separate in contrast because the raised lines of one pattern shield the white paper from view while between the lines of the other pattern the paper remains visible. The intaglio relief is uncopiable so that the copies will entirely lack the optically variable effect.
U.S. Pat. No. 5,301,981 discloses a copy preventing sheet-form screen having indicia printed thereon which is readable when the document is viewed obliquely. The information is unintelligible when copied by the conventional, perpendicularly aligned, phototransferance techniques such as photocopying, offset printing, and facsimile transmission.
Screen angle modulation and frequency modulation are additional techniques employed in document security systems. In a screen angle modulation system, the spatial frequency of photodiode arrays of common scanners cause warning indicia printed on a second portion of the document to become visible. Any form of warning indicia or disturbance may be utilized to take the form of an obvious message, such as "void" or "false". U.S. Pat. No. 4,582,346 discloses a security document that employs the principal of screen angle modulation. Additional examples are disclosed in U.S. Pat. Nos. 5,171,040 and 5,297,815. To have an optimum effect documents employing screen angle modulation require wet offset printing with printing accuracies in the microrange.
Iridescent security devices utilize multistate optical characteristics of materials that change dramatically when they are exposed to optical radiation at preferred wavelengths. The visual effects of such changes is a change of visible color. U.S. Pat. Nos. 5,087,507 and 5,338,067 disclose dye coating systems which exhibit specific variable optical characteristics with specific reduced response times when exposed to the switching activation radiation. The visual effects of such changes is a change of visible color. For example, one color will be essentially transparent in a natural state and convert into a deep blue color when exposed to long ultraviolet or short wavelength blue radiation. The color copier or any other four color reproduction system cannot recreate the iridescent effects of a multilayer composite system that provides multistate optical density at different optical wavelengths.
A document security system, as disclosed in U.S. Pat. No. 4,420,515, which relies upon latent images on the substrate provides the document with the characteristic of a changing relationship between the contrast of the image portion with respect to the background as a function of changing of view. A latent image on a substrate is non-reproducible by an electrostatic copier. When the original is copied, the copy is black due to random diffraction of light upon activation of a photocopy light source. The light is scattered to prevent the transmission of a focused reflected image to the copier drum.
U.S. Pat. No. 4,303,307 discloses a security document that copies black by providing a paper substrate including a dielectric layer formed of an electrically non-conducting material. A plurality of electrically conducting reflective spherical particles are embedded within the dielectric layer. The particles scatter the incident light and divert the transmission of reflective light away from the receiving elements of a 0/45.degree. specular illumination-type photocopier. Thus, the photocopying process is corrupted by dispersing the light in a pattern to prevent the transmission of a focused, reflected image to the photocopier drum. Light incident from the illuminating lamp of the copier is directed away from rather than toward the copier drum, preventing copying of the original document.
While there are many known security methods and devices for preventing the unauthorized photocopying of documents, as well as forging and counterfeiting negotiable instruments, the known devices and methods are produced by techniques that require advanced technology at a considerable expense. The security devices are limited in use, such as holograms on credit cards or drivers licenses or the incorporation of optically variable devices in currency and other negotiable instruments. Most of the optically variable devices described above are prepared by laminating a number of different materials to a substrate. The manufacturing processes are cost prohibitive for conventional commercial use and are not compatible with conventional printing techniques such as lithography, dry litho, laser printers, bubble jet printers, ink jet printers, electrostatic toner printers or halftones using commercially available printing inks. Therefore, there is need for a security document that is applicable to a wide variety of uses and inhibits the generation of copies by the many commercial reproduction devices available for both the 0/45.degree. specular illumination type and the diffuse illumination type which include electrostatic copiers, CCD scanners, laser scanners, glow lamp scanners, cameras, enlargers, contact frames, facsimile machines and the like devices.